Optical beam splitters, whose function is to guide the illumination light emitted from a light source onto the sample to be imaged, and to guide the detected light proceeding from the sample onto a detector, are often used in microscopy. The illumination light serves, for example in fluorescence microscopy, to excite fluorescent radiation. In this function it is hereinafter also referred to as “excitation light.”
An optical beam splitter is as a rule embodied so that it transmits light of a predetermined wavelength region and reflects light of another wavelength region. It should therefore be designed especially for the combinations of wavelengths of the excitation light and the fluorescent light that are used in the respective microscope. This is disadvantageous in terms of the broadest possible applicability of such a beam splitter.
Acoustooptical beam splitters, on the other hand, which are used for example in confocal microscopy, are adjustable within comparatively broad limits to the wavelengths to be reflected and transmitted by them. For example, acoustooptical beam splitters of this kind can be used in conjunction with so-called white light laser sources in order to select individual excitation wavelengths, as desired, from a broad wavelength spectrum. Adjusting an acoustooptical beam splitter to specific wavelengths requires a certain technical outlay, however, for example in the form of corresponding programming of the beam splitter by the application of electric fields of different frequencies. This is particularly disadvantageous when the adjustment of the beam splitter needs to be modified frequently, as is the case, for example, when using solid-state laser light sources whose output wavelengths exhibit comparatively severe temperature drift. It is also especially disadvantageous if the wavelength or wavelength spectrum of the laser light source is unknown and/or variable. Acoustooptical beam splitters are moreover comparatively expensive.